This Little Amp Is Trying to Kill Me and My Girlfriend

Discussion in 'Shock Brother's DIY Amps' started by ArcticWhite, May 22, 2020.

  1. ArcticWhite

    ArcticWhite Tele-Holic

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    I built this little 7 watt guitar amp out of a Silvertone organ amp.

    Last night, I plugged it into an ungrounded, three prong outlet in my garage. My girl came in and I put my arm around her while my left hand was holding the strings. I felt the small of her back buzz, and I immediately let go.

    Unplugged and started testing things.

    The outlet is wired correctly with the exception of the lack of a ground. Checked polarity with a plug in tester.

    I measured the voltage from the strings to known ground at about 55V AC.

    Opened the amp up and did a visual inspection. Can't see anything wrong.

    The amp works fine. But there's a problem somewhere.

    For comparison, I plugged my Princeton Reverb into the same outlet with the same guitar&cord combo, and there is no measurable voltage on the strings.

    20200522_150503.jpg

    20200522_145144.jpg 20200522_145220.jpg 20200522_145234.jpg
     
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  2. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    EDIT:
    Try lifting the PT off the chassis. Maybe a short from the winding.

    Look for frayed insulation on AC wires.
     
    Last edited: May 22, 2020
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  3. ArcticWhite

    ArcticWhite Tele-Holic

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    (Edit: Because my response doesn't make sense in context anymore.)

    Center tap is here, soldered to one of the cap can tabs.

    20200522_163301.jpg
     
  4. printer2

    printer2 Poster Extraordinaire

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    Your PT secondary, I take it the center tap is returned to the chassis?
     
  5. Dacious

    Dacious Poster Extraordinaire

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    Green-eyed jealousy?
     
  6. ArcticWhite

    ArcticWhite Tele-Holic

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    Do you mean an internal short, or one of the leads?
     
  7. jimbo735

    jimbo735 Tele-Afflicted

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    Is it a optical allusion or does it look like on the input jack the guitar cord tip would touch that resistor when plugged in.

    Is the amp in a cabinet your on a cement floor Im assuming,thats always trouble from what I have experienced in the past.
     
  8. Badside

    Badside Tele-Meister

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    That big green wire, that's the mains ground right? Doesn't look it's connected to the chassis no? Should be.
     
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  9. tubegeek

    tubegeek Tele-Holic

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    Ordinarily that'd be a huge clue, but the OP's outlet isn't grounded! So it would not make a difference in this specific instance.

    (It's grounded through the attachment of the last terminal on the strip to the chassis but you can't see it in the picture.)
     
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  10. tubegeek

    tubegeek Tele-Holic

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    What's the insulated RCA jack for? Line out?
     
  11. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    An internal short to the plates of the PT which are bolted to the chassis could do it.

    One of the leads carrying AC could be frayed and make contact either in the PT or as they pass through the chassis.
     
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  12. ArcticWhite

    ArcticWhite Tele-Holic

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    Tremelo footswitch jack
     
  13. ArcticWhite

    ArcticWhite Tele-Holic

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    It's connected to the tab of the can. Solid. 20200522_164510.jpg
     
  14. ArcticWhite

    ArcticWhite Tele-Holic

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    I did try testing the transformer leads for shorts and didn't find anything.
    I try isolating it.
     
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  15. RodeoTex

    RodeoTex Poster Extraordinaire

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    Is one side of the PF xfmr grounded, or at least to the neutral?
    I'm suspecting 0v is floating.
    This is very '60s. It may work out for you.
     
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  16. RollingBender

    RollingBender Tele-Afflicted

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    Rubber grommets in every chassis hole which wired pass through.
     
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  17. ArcticWhite

    ArcticWhite Tele-Holic

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    It's soldered to a grounded tab on the cap can.
     
  18. GoldDeluxe5E3

    GoldDeluxe5E3 Tele-Afflicted Silver Supporter

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    Sorry, I keep reading your original post, and you say "ungrounded, 3-prong outlet".

    If you plug a 3 prong amp plug into an ungrounded socket, set the amp on your garage floor, your girlfriend comes in to visit and she's barefooted, you're holding your strings of your guitar plugged into the ungrounded amp, and you touch her back, I wouldn't be surprised if you feel a shock. You shouldn't be rummaging through your amp circuit to find an electrical problem.

    It can only be one thing. Your girlfriend seems to have an electrical malfunction.
     
    Last edited: May 22, 2020
  19. mfguitar

    mfguitar Tele-Afflicted

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    You really need to have that garage wiring checked out :)
     
  20. ArcticWhite

    ArcticWhite Tele-Holic

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    I did not say "ungrounded 3-prong plug". You did.
    Intentionally misquoting people is rude.
    Delete your post.
     
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